Improved Ionosphere D-region Specification for High Frequency Propagation

HF and VLF skywave propagation depend on knowledge of the altitudinal structure of the ionosphere for reliable system operation. The D-region ionosphere, a region typically between 60-90 km altitude above the Earth’s surface, is particularly challenging to predict with sufficient fidelity for skywave mission planning because the region is difficult to measure with in situ measurements. Remote sensing methods provide a limited dataset from which to develop detailed physical and assimilative models that can reliably produce the regional behavior on a day-to-day basis. Operational models like the D-Region Absorption Product (D-RAP) produce maps of outage areas as a function of radio frequency (RF) to inform HF system users of anticipated degradation or blackouts due to radiation increases. These predictions primarily address HF absorption due to solar X-ray flares at low and mid-latitudes and do not address the VLF propagation use case. While solar X-ray flares are important effects with Navy relevance, the effects of flares typically only last for a few hours and occur 10-20 times a year. For sufficient use in HF absorption and VLF propagation applications, a D-region ionosphere is required to capture seasonal, solar cycle, and day-to-day variability at all latitudes in addition to solar flare phenomenology. The intended capability is improved D-region specification and forecast in the 60-70 km and 80-90 km altitude regions for VLF propagation and HF absorption, particularly day-to-day variability and in the high latitude regions at night. Climatological models like the International Reference Ionosphere (IRI) produce an altitude profile at D-region altitudes. The IRI model is based on about a hundred historical rocket profiles, so it captures some of the basic day/night, latitudinal, and seasonal variations in the D-region ionosphere but the model does not capture day-to-day variations or solar X-ray flare events. The model may be sufficient for some HF propagation applications but has been shown to be severely deficient in the high latitude region and during nighttime. In addition, the observations used to create the model do not accurately capture the shape of the lower D-region, which is required for VLF propagation applications. The Next-generation Ionospheric Model for Operations (NIMO) is an assimilative geospace model developed to address operational USN space weather needs in the ionosphere. NIMO harnesses contributions from both near real-time data and state-of-the-art implementation of ionospheric theory to provide hindcasts, nowcasts, and forecasts for operational or research purposes. NIMO is currently configured to assimilate various types of electron density measurements through the Ionospheric Data Assimilation Four-Dimensional (IDA-4D) data assimilation schema. The D-region is not included in the data assimilation process but is simply added in post processing to the bottom of the electron density grid. The D-region model includes influence from changes in the solar conditions, but uses a climatological model for the neutral atmosphere, so it does not capture the day-to-day variability that is seen in the ionosphere. Key performance Metrics/Goals: Techniques to improve the global specification and forecast of the D-region ionosphere is sought with particular emphasis on the low latitudes (< 30 degrees) and high latitudes (> 60 degrees). Note: Any measurement techniques should provide measurements or any model should ingest measurements of the D-region and demonstrate improvement over existing models in two key altitude regions: 60-70 km altitude and 80-90 km altitude, which are required for VLF propagation and HF absorption, respectively. Key parameters include 1. Specify electron density in the 60-70km and 80-90km regions within 10% error. Alternatively, an improvement 75% of the time over existing models is sought. 2. Specification at a temporal cadence of 5-15 minutes 3. Specification at a spatial resolution of 5 degrees in latitude and longitude Technical concepts may include developing prototype concept improvements to measurement techniques and systems, developing improvements to data exploitation and modeling, or a combination of the two.